I've implemented an OpenID controller using Dnoa. I was using the approach found in the RP MVC template for dnoa, which returns IAuthenticationRequest.RedirectingResponse.AsActionResult() from the controller action. However, I've now found the controller very difficult to test using Moq because of this method.
I changed the controller to instead invoke IAuthenticationRequest.RedirectToProvider() and return null for the ActionResult, which seems to have the same effect. RedirectToProvider should be easier to mock, but is it appropriate to return null from the controller in this case?